The principle class of Phase I drug-metabolizing enzymes is called "cytochrome P-450." Subsets of this class include at least three P-450 gene families inducible by 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD; called in the lay press "dioxin"), phenobarbital, and steroids, respectively. Presumably the induction process for all three gene families is governed by receptors. The TCDD inducible P-450 gene family is controlled by the Ah receptor, and the entire genetic system is referred to as the Ah locus (aromatic hydrocarbon responsiveness). This laboratory has worked principally on the TCDD-inducible P-450 gene family among inbred mouse strains and tissue culture lines. This gene family is composed of two major genes, P1-450 and P3-450. The P1-450 and P3-450 proteins were purified from 3-methylcholanthrene-treated C57BL/6N mice, and polyclonal antibodies were developed. These antibodies were used for polysome immunoadsorption to purify the corresponding messenger RNAs. By means of the Okayama-Berg plasmid vector, full-length cDNA clones were isolated and sequenced. A genomic-DNA library from C57BL/6N liver was also constructed. The P1-450 and P3-450 genes were isolated and sequenced, including all six introns and more than 1000 base pairs in both the 5' and 3' flanking regions. The protein sequences, deduced from the nucleotide sequences, allow us to conclude that the TCDD inducible and phenobarbital-inducible P-450 gene families diverged from a common ancestral gene more than 200 million years ago and that the homologous P1-450 and P3-450 genes separated from each other at least 65 million years ago. Human P1-450 cDNA and genomic clones are also being isolated and sequenced. We hope to develop an assay, based on recombinant DNA technology, to assess the human Ah phenotype. Such an assay may predict who is at increased risk for certain types of environmentally-caused birth defects, cancers, and toxicity.